This invention relates generally to radio frequency circuits and more particularly to radio frequency mixer circuits.
As is known in the art, there exists a trend in radio frequency (RF) transmitting and receiving systems towards operation in high frequency bands such as the millimeter wave frequency band. At higher frequencies, insertion losses in RF receiver components such as limiters, mixers and the like are increased and, therefore, as a result, receiver sensitivity is decreased. It is also known that RF receivers operating in the millimeter wave frequency band require mixers capable of operation in this frequency band. A mixer is a device which converts RF power at one frequency into RF power at a different frequency where it can be processed efficiently. Generally, a received radio frequency (RF) signal and a local oscillator (LO) signal are fed to the mixer circuit to provide an intermediate frequency (IF) output signal having a pair of frequency components, often called sidebands, equal to the sum and difference of the frequencies of the RF signal and the LO signal. Mixer circuits that operate in this way are said to be fundamentally pumped mixers. In most receiving systems, the lower sideband (the down converted product) is used, whereas in transmitting systems, the upper sideband (the upconverted product) is selected. The ability to shift a signal in frequency with minimal added noise or distortion is important because the properties of radio frequency signal processing circuits such as amplifiers, filters, and detectors are generally frequency dependent. In order to perform these signal processing functions optimally, it is often necessary to shift signals to the frequencies where the signal processing circuits can perform their functions best.
In the area of microwave/millimeter wave technology, there has been increased emphasis placed on the development of so-called hybrid circuits which utilize a combination of monolithic and printed circuit technology such a microstrip. Hybrid circuits typically have the active portion of the circuit fabricated as a monolithic microwave/millimeter wave integrated circuit (MIMIC) while other components such as filters, which are difficult to fabricate as a monolithic component, are fabricated using printed circuit transmission lines. The monolithic circuit is then disposed on the printed circuit transmission lines resulting in a so-called hybrid circuit. Hybrid microwave/millimeter wave integrated circuits (HMIMIC) offer the promise of low manufacturing costs and low variations in electrical characteristics from unit to unit. Single function circuits, such as mixers, have been developed and fabricated using these HMIMIC techniques.
With mixers and other frequency conversion circuits, such circuits include a device having a nonlinear transfer function that can be expressed as: EQU I=f(V)=a.sub.0 +a.sub.1 V+a.sub.2 V.sup.2 +a.sub.3 V.sup.3 + . . . +a.sub.n V.sup.n
where I and V are the device current and voltage, respectively. For a mixer, voltage applied to the nonlinear device, V(t), is a composite of the radio frequency (RF) signal (V.sub.RF Sin .omega..sub.RF t), where .omega..sub.RF is the RF signal frequency in radians, and the local oscillator (LO) signal (V.sub.LO sin .omega..sub.LO t), where .omega..sub.LO is the local oscillator signal frequency in radians, in the form V(t)=V.sub.RF (sin .omega..sub.RF t)+V.sub.LO (sin .omega..sub.LO t). The LO signal is sometimes called the "pump" waveform, and the mixer is often said to be pumped when the LO signal is applied to the non-linear device. Application of the RF signal and Lo signal to the non-linear device results in a composite output signal, S.sub.o, having an infinite series of mixing products given by ##EQU1## As mentioned above, the mixing product or sideband desired at the output is the sum and difference of the signal inputs (.omega..sub.RF .+-..omega..sub.LO) and is called the intermediate frequency (IF).
One problem with mixers is that a nonlinear device generates many mixing products other than the desired one. These products include harmonics (n.omega..sub.RF, m.omega..sub.LO) of the original input signals, .omega..sub.RF, .omega..sub.LO where n and m are positive integers. Additional products further include intermodulation products of the harmonics (m.omega..sub.LO .+-.n.omega..sub.RF) and a DC output level. It is generally necessary to filter the input signals and other mixing products from the mixer to extract the desired intermediate frequency signal and reject or terminate the undesired frequency signal components.
As mixer circuit applications extend to higher frequencies, such as the millimeter-wave region mentioned above, additional problems arise. For example, in order to maintain a relatively low IF signal frequency, the frequency of the local oscillator source required for mixing with the received RF signal frequency concomitantly increases. As the LO frequency requirements increase, the ability of LO signal sources to meet suitable power, bandwidth and noise requirements becomes increasingly difficult to achieve. Therefore, the relative cost of the LO source will increase while the relative reliability of the LO source will decrease.
One solution to this problem is to use the so-called subharmonically pumped (SHP) mixer. The SHP mixer operates similarly to the fundamentally pumped mixer, except that the SHP mixer is pumped at a submultiple of the LO frequency normally used with a fundamentally pumped mixer.
In the millimeter-wave frequency region, a common subharmonically pumped waveguide mixer is the so-called balanced subharmonically pumped mixer which includes two pairs of anti-parallel diodes as mixing elements. Each pair of diodes has a cathode of a first one of the diodes connected to an anode of a second one of the diodes and a cathode of the second diodes connected to an anode of the first diodes. In one embodiment of a balanced configuration, the two pairs of anti-parallel diodes are disposed in parallel on a thin substrate such as quartz. The substrate is disposed in the transverse plane of a waveguide, so that the diodes intercept RF energy in the waveguide. An LO signal is supplied to the two pairs of anti-parallel diodes through a power divider. Each pair of diodes produces an IF output signal in response to applied RF energy in the waveguide and the applied LO signal. The IF signals, properly phased, are fed to a pair of input ports of a quadrature coupler with one of the remaining ports providing an output signal and the last port being terminated in a characteristic impedance as is known.
Several problems exist with the balanced subharmonic mixer circuit approach mentioned above. First, since the anti-parallel diode pairs are in parallel, the impedance mismatch between the diode pair and the RF signal and IF signal load impedances is doubled. To achieve a good match to the anti-parallel diode pairs, the waveguide impedance must be kept low. To lower the waveguide impedance, the waveguide height must be made small. At millimeter wave frequencies where the waveguide dimensions are already relatively small, it becomes substantially more difficult to mount the diodes in the waveguide. A second problem with the balanced subharmonic mixer circuit is the requirement to have power divider circuits which further complicate the design, and increase the loss of the mixer circuit.
Another example of a subharmonically pumped mixer is the single ended subharmonically pumped mixer which includes one pair of anti-parallel diodes. As mentioned earlier, application of a composite waveform of the RF signal and LO signal provides an output having frequencies that include m.omega..sub.LO .+-.n.omega..sub.RF. When the same composite waveform is applied to a symmetric anti-parallel diode pair, signals having frequencies of m.omega..sub.LO .+-.n.omega..sub.RF in which m+n are even (called even-order harmonics) including .omega..sub.RF -.omega..sub.LO and .omega..sub.RF +.omega..sub.LO and the DC term flow only within the diode loop. Signals having frequencies in which m+n are odd (odd-order harmonics) including .omega..sub.RF -2.omega..sub.LO flow out of the diode pair. A typical implementation of a single-ended SHP mixer includes one anti-parallel diode pair and three bandpass filters. Bandpass filters are used to filter out undesired signals at each mixer port.
A mixer having an anti-parallel diode pair with a first electrode fed by an RF signal and a second electrode fed by an LO signal would typically have a bandpass filter at the first electrode for filtering undesired signals in the RF signal path and a bandpass filter at the second electrode for filtering undesired signals in the LO signal path. A third bandpass filter would be used for filtering undesired signals in the IF signal path.
The problem of impedance matching between the diode pair and the waveguide in the balanced SHP waveguide mixer mentioned above is somewhat lessened in the single-ended SHP waveguide mixer since only one diode pair is required in the single-ended SHP mixer. Nevertheless, it remains a problem to match the naturally low diode impedance to the naturally high waveguide impedance.
A further problem with using a bandpass filter at each mixer port is that the conversion loss of the mixer increases because the bandpass filters have relatively high insertion loss characteristics particularly at millimeter wave frequencies.
A further problem with this implementation is that bandpass filters are difficult to fabricate at millimeter wave frequencies due to the sensitivity of the bandpass filters to fabrication tolerances.